igb: override two checker warnings

The igb PTP code was using htons() on a constant to try to
byte swap the value before writing it to a register. This byte
swap has the consequence of triggering sparse conflicts between
the register write which expect cpu ordered input, and the code
which generated a big endian constant. Just override the cast
to make sure code doesn't change but silence the warning.

Can't do a __swab16 in this case because big endian systems
would then write the wrong value.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>